Over the past several years, a significant increase in recording density in thin-film media has been achieved, and there is a continuing effort to increase recording density further.
A number of magnetic properties in a thin-film media are important to achieving high recording density, including high coercivity and remanence, and low flying height. The latter property is important because as the read/write head is moved closer to the disk, there is less overlap of voltage signals in adjacent magnetic domains in the disc, thus permitting recording density to be increased.
Heretofore, disks having high coercivity and remanence characteristics have been prepared by sputtering a thin magnetic film on a metal substrate, typically an aluminum substrate. Prior to sputtering, the substrate is plated with a selected alloy plating, such as a nickel/phosphorus coating, to achieve a requisite surface hardness, and the plated substrate is polished to remove surface irregularities. Before applying the sputtered layers, the surface of the substrate is textured to create a roughened surface characterized by submicron surface irregularities. The roughened surface reduces stiction between the disk and head by reducing surface contact between the two.
There are several surface-texture properties which may affect stiction and disc wear relating to start stop-cycles. One is surface roughness, which provides a measure of peak-to-valley distances in a plane. Roughness is commonly expressed as arithmetic average value of absolute distances above and below a reference plane along a line. In general, the stiction coefficient tends to drop significantly at a average mean roughness value above about 20-50 .ANG..
Another surface-texture property is summit density, which provides a measures of peak density in a given surface area. A higher summit density may lower stiction by lowering the total surface contact between peaks in a textured surface and the surface of the read-write head.
The degree of isotropy of the texture in the plane of the surface can also influence stiction coefficient. In a substantially isotropic texture, both the roughness and summit density features of a surface are relatively uniform in any direction of the plane, and generally higher summit densities can be achieved. An isotropic texture also reduces any position-dependent variation in stiction, which can lead to increased wear in the disc surface.
Heretofore, in metal substrates coated with a metal plating, such as a polished nickel-phosphorus coating, mechanical abrading methods have been used to achieve a desired surface texture. In this method, the substrate is rotated in a abrasive-particle slurry, while a pad is pressed against the substrate surface, or portion thereof, with a selected abrading force. To increase surface roughness it is necessary to apply a greater pad pressure against the rotating disc, or to employ larger-size abrasive particles, or a combination of both. However, applying a greater abrading force increases the possibility of damaging the plated surface mechanically, which can lead to disc failure. The size of abrasive particles which can be employed is also limited by the reduction in peak density which occurs with larger-size particles. Peak density is also limited, in mechanical abrasion, by the highly anistropic texture, which favors high peak density in a radial direction, but relatively low peak density in an angular direction.